Successful treatment of cancer by neutron therapy necessitates a knowledge of the degree of risk of normal tissue sequelae associated with the cure of the tumor. This will be expressed as the ratio of the amount of photon irradiation which results in several specific late effects (especially carcinogenesis) to the amount of neutron irradiation which results in the same specific effects (RBE values). The aim of this investigation will be to obtain these values for sequelae in the brain, spinal cord and lung. Currently fast neutrons are being used by the Middle Atlantic Neutron Therapy Association (as well as several other centers) in the treatment of cancer patients which include the Brain, spinal cord and lung within the treatment area. Except for the enclosed data, the chronic effects of the MANTA beam on these organs have not been studied, nor has a comparable investigation been performed elsewhere. Experiments based on a pilot study have been designed to investigate the late effects of 15 MeV neutrons on these organs in beagles. The ongoing study, which has utilized the same fractionation schedule used clinically, will continue to evaluate the sequelae utilizing functional and quantitative methods. Late effects on brain and cord will be measured by quantitative studies including neurological function examination, sensory and visual evoked responses, onset of neurological deficits (paresis, paralysis) and quantitative histopathological studies. Late effects on lung tissue will be measured by radionuclide evaluation of pulmonary function through regional ventilation, perfusion, ventilation-perfusion ratios, and radioaerosol clearance as well as quantitative histopathologic studies. The onset, incidence and degree of malignancy of neoplastic tissue will be documented and evaluated by radiography, computerized axial tomography and histological evaluation. Previous evaluations of sequelae of neutron therapy have failed to use clinical exposure schedules or to employ quantitative methodology subject to statistical analysis.